libBacktraceOffline: try both .debug_frame and .ARM.exidx on arm.

Currently, libBacktraceOffline doesn't try .ARM.exidx if a function
appears in .debug_frame. This make it can't unwind through functions
appear in both .debug_frame and .ARM.exidx, but having dwarf instructions
not accepted by libunwind.
This patch fixes this by trying first .debug_frame then .ARM.exidx on arm.

Bug: http://b/69383534
Test: run libbacktrace_test on arm.

Change-Id: Ib95dd56d5cc123a20948e880b51b28ddc04b4a6e