ueventd: bump UEVENT_MSG_LEN to 8192

The previous size, 2048, is only the size of the 'environment' for the
uevent message, but doesn't include the <action>@<dev path> portion.
The <action> portion has a max length < 10, but the <dev path> portion
is unbounded.

8192 should be plenty to capture all of these parameters.

Bug: 161580785
Test: ueventd still works
Change-Id: I6de6fd3a444ac91b3b4df154097abde3696e21b3