What looks like is happening here:
* Android decides to sync. There is no Fennec process, so it starts one.
05-03 17:19:02.454: I/ActivityManager(101): Start proc org.mozilla.fennec_aurora for service org.mozilla.fennec_aurora/org.mozilla.gecko.sync.syncadapter.SyncService: pid=823 uid=10064 gids={3003, 1015, 1006}
* It has to publish all of Fennec's CPs in order to do so.
05-03 17:19:02.524: I/ActivityThread(823): Publishing provider org.mozilla.fennec_aurora.db.browser: org.mozilla.fennec_aurora.db.BrowserProvider
05-03 17:19:02.594: I/ActivityThread(823): Publishing provider org.mozilla.fennec_aurora.db.tabs: org.mozilla.fennec_aurora.db.TabsProvider
05-03 17:19:02.614: I/ActivityThread(823): Publishing provider org.mozilla.fennec_aurora.db.formhistory: org.mozilla.fennec_aurora.db.FormHistoryProvider
* SyncAdapter calls AccountManager.get to get your credentials, and waits for the callback:
05-03 17:19:02.674: D/SyncAdapter(823): AccountManager.get(org.mozilla.gecko.GeckoApplication@462958a0)
* 30 seconds later, it times out without ever getting the callback.
05-03 17:19:42.758: W/ActivityManager(101): Timeout executing service: ServiceRecord{46549af8 org.mozilla.fennec/org.mozilla.gecko.sync.syncadapter.SyncService}
GeckoApp shouldn't be the context here, which is kinda weird, but other than that this looks like an "Android sucks" bug -- we're sitting and waiting on an Android system call that never returns.
Would need more reports to think that this is a real issue.

(In reply to Gian-Carlo Pascutto (:gcp) from comment #2)
> Could there be some interaction with Sync migration here? This was the users
> first run.
Android will try to sync when an account is created, yes, including through migration. I don't know why that would cause the account query to time out.

Adrian, your log suggests that you have more than one version of Firefox installed.
05-04 10:27:44.660: DEBUG/PackageManager(2706): Scanning package org.mozilla.fennec
05-04 10:27:44.660: DEBUG/PackageManager(2706): Shared UserID org.mozilla.fennec.sharedID (uid=10134): packages=[PackageSetting{40d5f9e0 org.mozilla.fennec/10134}, PackageSetting{40611578 org.mozilla.fennec_aurora/10134}]
05-04 10:27:44.660: INFO/PackageManager(2706): Package org.mozilla.fennec codePath changed from /data/app/org.mozilla.fennec-1.apk to /data/app/org.mozilla.fennec-2.apk; Retaining data and using new
Could you try again with only one version installed?

(In reply to adrian tamas from comment #11)
> I am able to reproduce the issue using only one Firefox app installed
> (Nightly).
Can you provide a log with only a single instance installed?
Do you have STR?

Steps to reproduce:
1. Install Nightly XUL 15.0a1.
2. Set up a profile sync on the device and wait for the data to be synced.
3. Download and install Nightly Native 15.0a1
4. Tap the Open button and wait for Nightly to start.
Actual:
After a few minutes I get a "Force close" dialog. Tapping on Wait to continue setting up Nightly just leaves the app standing at the setup screen.
The issue is only reproducible with this one profile. from the previews investigations I believe the data is not a factor only this profile. I can't determine any differences between profiles to narrow down the issue unfortunately.

Curiosities:
05-08 10:37:21.962: I/ActivityManager(1359): Process com.android.settings (pid 8253) has died.
Profile migration takes 30 seconds.
After we create the account, a bunch of other system accounts get the chance to run.
Adrian, can you reproduce this if you uncheck every box in your Android "Accounts & sync" screen prior to upgrading? The only thing I can imagine is happening here is that we're waiting for the rest of Android to run.
If you're literally only able to reproduce this with one existing profile, then I'm inclined to close this INCOMPLETE, and say "blame Android"...