Reduce Binder transaction size in getAllProperties

getAllStrings makes a Binder transaction to retrieve all flag keys and values.
The transaction is becoming prohibitively large, so switch to making a Binder
call to retrieve the namespaces first, then call getStrings with each namespace.

Change-Id: Icb3d946a2cdf9c4497ab0605b4fe9101ed67598d
Flag: android.provider.reduce_binder_transaction_size
Test: m
Bug: 362652574