Use fallback linker allocator in trace_handler.

There is more than one function that can allocate in the fallback
path. Therefore, make sure that all functions that can allocate have
switched to the fallback linker allocator before allocating. This
is mostly a problem for the trace_handler function call, which iterates
over all known threads and gets backtraces for them.

Add a ScopedUseFallbackAllocator class to do the switching to make it
easier to have the same code everywhere.

Add two tests to verify that no allocations are occuring during
a tombstone or backtrace on the fallback path. These tests are not
comprehensive since they can't verify that the linker allocator is
using the fallback allocator, but they are better than nothing.

Remove the debuggerd_fallback_tombstone() function since it only
enables the linker callback and then calls engrave_tombstone_ucontext().
The enabling is now done with the ScopedUseLinkerAllocator.

Restructure the case where sending the signal to a thread fails so
that the fallback allocator can be enabled properly.

Bug: 359692763

Test: Ran debuggerd -b <PID> and debuggerd <PID> on a process that
Test: goes throught the fallback path.
Test: Unit tests pass along with two new tests.
Test: Forced a fallback process to crash and verified tombstone generated.
Test: Instrumented the linker allocator and verified that the trace_handler
Test: function never calls the normal linker allocator as it runs.
Change-Id: I2710921076634eac97f41bec8c3a29c1d75ae5ec