Changed BugReportReceiver to send zipped bugreports.

When BugReporterReceiver receives a android.permission.DUMP, it will handle the bugreport file (extra android.intent.extra.BUGREPORT) depending on its format:

 - If it's a plain-text file (extension .txt), it will create a zip file with the plain-text report renamed as bugreport.txt.
 - If there is an error creating the zip file, the plain-text bugreport will be sent instead.
 - If it's not a plain-text file, it fill send the file as is (so if 'dumpstate' is later changed to generate a zipped file directly, nothing has to be done here).

Note that only the bugreport itself is included in the zip file, the screenshot is still sent separately (extra android.intent.extra.SCREENSHOT), so the receiver of the bugreport intent can display a screenshot thumbnail without unzipping the other file (which is useful when sending the bugreport through GMail, for example).

BUG: 20447313
BUG: 21868658
Change-Id: I3fa1e0c89190bfe6fa5c418f0f01ce1fb376f537